The present invention relates to detectors for automatically sensing the presence of a dangerous condition and energizing appropriate protective apparatus. Many types of detectors are known for sensing various dangers or potentially dangerous conditions. Pressure and temperature detectors are well known as are optical flame and smoke detectors. Fire detection by sensing emitted ultraviolet radiation is also well known.
In the design of such detectors and more particularly in the design of explosion detectors, two conflicting design criteria operate. The first is minimalization of the reaction time in which an output indication signal can be provided to protective apparatus and second is reliability in the presentation of false alarms. Particularly with respect to explosion protection the short reaction time is critical since remedial measures against most types of explosion must be taken within approximately 100 msec of the onset thereof in order to prevent serious damage to life and property. Reliability is also critical since such explosion detectors are often coupled to automatic explosion prevention apparatus and it is extremely desirable that such apparatus not be operated except in the case of actual need. An example of fire and explosion suppression apparatus suitable for use with detector apparatus of the type which form the subject matter of the present application is the apparatus described in our copending U.S. patent application Ser. No. 902,610 filed concurrently herewith and of common assignment herewith.
A number of fire and explosion detection system have been proposed.
Two relevant examples are illustrated in U.S. Pat. Nos. 3,825,754 and 3,931,521. U.S. Pat. No. 3,931,521 describes a dual spectrum infra-red fire detector which is activated by the coincident receipt of radiant energy in 7-30 micron spectral band and in 0.7-1.2 micron spectral band. The long wave length spectral band is detected by using a thermal detector such as a thermopile. The detector system described in U.S. Pat. No. 3,931,521 suffers from the disadvantage that the short wave length detector is responsive to light in the visible band which is transmitted through the atmosphere, and the long wavelength detector operates in a region of relatively high noise. Thus the device operates at a relatively low sensitivity threshold of operation.
U.S. Pat. No. 3,825,754 describes a dual spectrum infra-red fire detector similar to that described in U.S. Pat. No. 3,931,521 and also comprises a three channel infra-red radiation detection system for distinguishing between large explosive fires and large explosions which cause no fire. The system described in U.S. Pat. No. 3,825,754 shares the disadvantages of the system described in U.S. Pat. No. 3,931,521 as discussed hereinabove.
U.S. Pat. No. 3,665,440 shows a combination ultra-violet and infra-red detection system which provides an output only in the absence of ultra-violet radiation during the receipt of infra-red radiation. Such a detector system is not suitable for use in detecting incipient explosions.
U.S. Pat. No. 3,653,016 shows a combination infra-red light detector and ultra-violet light detector coacting as a fire discrimination system. Since visible light is detected the false alarm rate of such a detector is increased when visible light is present in the detection environment.